for the shower, i plan on using a kohler shower system... the requirement says that 2" is fine... but in case of overflowing... i might just go 3"... due to the amour of flow the shower heads can generate.

to that same 4" main waste and vent, there is a washing machine and sink connected to it in the next room.

this bathroom is on the second floor and above it there is the roof. on the 1st floor, some 12' below (1905 house) the laundry room is the kitchen with a sink and a dishwasher.

I STRONGLY RECOMMEND you consult a Master Plumber or Plumbing Engineer to Evaluate and DESIGN this modification for your DWV system. You could then (if you feel qualified) possibly perform the work yourself.

It appears obvious you are presently wet-venting if your diagram is complete, and that your present toliet location is near the DWV stack, and all fixtures on the same long 14' wall.

I also find it more than concerning that your joist diagram has the joists running along this 14' wall, and that your bathroom is 7' deep and you only have 2 joists diagramed - and that you propose to pass your venting THROUGH the joists, yet make no diagram regarding the MAJOR structural corrections to be able to do this - even if you had 2x12" joists.

You WILL have to auxiliarly vent the far from the DWV main stack fixtures. Is your present bath tub where you propose a shower to be?

I can't imagine (if your diagram is to scale) that you have enough clearance (to ANY PLUMBING CODE) for the toilet footprint.

One also wonders if any of these walls are outside walls - where the door is, and where the window is.